In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and made part of this disclosure.
The fabrication of robust interfaces between electrodes and electrolytes that satisfy the application-dependent, electrochemical demands of various systems is one of the great challenges of interfacial science. In particular, irreversible reactions between non-aqueous electrolytes and metal oxides are often considered to be a rate-limiting step adversely affecting the overall performance of lithium-ion (Li-ion) batteries. Li-Ion batteries, in general, suffer irreversible capacity losses during high voltage operation due, in part, to corrosion of active materials in the acidic electrolyte as well as electrolyte decomposition. Small levels of moisture (˜20-100 ppm) present in battery-grade electrolytes can react with lithium salts (e.g. LiPF6) to generate hydrofluoric acid (HF). HF subsequently attacks the surfaces of electrode materials causing transition metal dissolution and migration to the negative electrode. Furthermore, surface layers can form as a result of electrolyte oxidation at high voltages which impede Li-ion diffusion.
Coating electrode surfaces with metal oxides such as Al2O3, MgO, ZnO, and TiO2 has proven to be effective in mitigating irreversible side reactions. These metal oxides coatings, however, are also susceptible to HF attack and may not be stable over long-term cycling, converting partially to metal fluorides when scavenging HF as reported earlier. Furthermore, byproducts of these reactions generate additional water in the electrolyte again making electrode/electrolyte interfaces unstable. In addition, the high electronegativity of fluorine results in strongly bonded cations relative to oxygen and should be beneficial in limiting unwanted surface reactions. In particular, AlF3-coated positive electrodes (LiCoO2, LiNi1/3Mn1/3Co1/3O2, Li[Li0.19Mn0.57Ni0.16Co0.08]O2, etc.) have demonstrated significant improvements in cycling stability and safety when AlF3 is applied via solution-based routes. The AlF3 layers provide some resistivity to HF attack, but are insulating resulting in decreased cathode performance.
However, wet-chemical processes, as well as standard physical vapor deposition (PVD), have limitations. Non-uniform thicknesses and/or compositional variation of coating layers can arise from the directional nature of deposition (PVD) or by the secondary heating steps required for solution-based routes. As thickness can alter the impact the layer has on the underlying cathode performance, the ability to finely tune and control thickness is important.